546 



J. 0RSKOV 



will gather on the front lens and obstruct \'ision altogether.) 

 In this way, the agar will preserve its shape for at least twenty- 

 four hours, even though subjected to several exanainations, 

 provided these be not of too long duration. 



Now, do not these repeated examinations involve a great 

 danger of contamination from the air? A risk there is, of course, 

 but it is apparently, insignificant. In the first place it can easily 

 be ascertained that only very few "alien" colonies will be seen 



Fig. 3 



Fig. 4 



to develop in the Petri dish by contamination from the air, in 

 spite of repeated examinations within forty-eight hours, and it 

 would be a stroke of very bad fortune if such a germ from the 

 air should settle just within the field of vision to be examined. 

 If this should occur, it would soon be discovered; it could only 

 escape detection in case a germ from the air dropped upon the 

 selected colony immediatelj^ before inoculation was undertaken 

 from it, and then it would most probably be disclosed in further 

 investigations of the bacterial species in question, as it would 



